Setting default file format when saving baked textures?
To change the default image file type produced by Blender, you actually want to change the image settings in the Render panel.

To change the default for a single file, go to the Properties>Render panel and scroll down to the 'Output' submenu. Then select the file format and BW/RGB/RGBA to your desired format.



If you want to set this as your default image type for all of your projects, first open a new file or open a new instance of Blender. Change the settings to what you would like to be default, then press Ctrl+U.

This changes your startup .blend file so that when you open future projects, they will come preloaded with these settings. It will not affect any of your old .blend files.